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Nokia and Microsoft's first flagship smartphone for the U.S., the Lumia 900, made its official debut at the 2012 Consumer Electronics Show in Las Vegas.

The new Windows Phone handset was first unveiled Monday by Nokia, and later that night Microsoft brought the new phone on stage in what was the final CES keynote speech from the tech giant best known for the powerhouse Windows PC operating system.

The Lumia 900 so far has been confirmed as running only on AT&T's 4G LTE network and picks up stylistically where the Lumia 800 left off, with an attractive rounded polycarbonate body and a flat, sliced-off-looking top and bottom.

However, the Lumia 900 will have a larger screen than the Lumia 800 -- up to 4.3 inches from 3.7 inches. The resolution of the display will remain 480 by 800 pixels, as is standard for all Windows Phone handsets.

The new Nokia will be offered from AT&T in either cyan or matte black and feature a 1.4-gigahertz Qualcomm processor, 512 megabytes of RAM, 16 gigabytes of built-in storage, an 8-megapixel rear camera that can shoot up to 720p video and a 1.3-megapixel front facing camera for video chatting.

The Lumia 900 will be thinner than T-Mobile's Lumia 710, a 0.45-inches-thick 4G phone I reviewed last weekend.

Nokia officials also told me at CES that the Lumia 800 is finally going to get a U.S. launch as well, but it will be sold only as an unlocked phone. That means the Lumia 800 will sell without part of the cost of the phone being eaten up by a wireless carrier's subsidy, which may put it in the $500-range, though Nokia declined to specify.

Microsoft and Nokia also had no details to offer on pricing or a release date for the Lumia 900. As soon as we can, we'll get the phone in our hands for a full review. In the meantime, check out our hands-on video from CES with both the Nokia Lumia 900 above; and photos and of the Lumia 900 and Lumia 800 after the jump.